Chains and channels in polynuclear copper(II) complexes with 2,3-bis(2-pyridyl)pyrazine (dpp) as bridging ligand; syntheses, crystal structures and magnetic properties

The preparation and crystal structures for three Cu(II) polynuclear, chain complexes with 2,3-bis(2-pyridyl)pyrazine (dpp) as bridging ligand are repo rted, [Cu(dpp)(H2O)(2)](n) (NO3)(2n). 2n/3H(2)O (1), [Cu(dpp)(H2O)(2)](n) ( CF3SO3)(2n) (2), and [Cu(dpp)(H2O)(2)](n) (BF4)(2n). 2n2H(2)O (3). For the latter compound the crystal structure at four different temperatures have b een studied. Variable-temperature magnetic susceptibility data and ESR meas urements of 1-3 and of the related copper(II) chain [Cu(dpp)(H2O)(2)](n) (C lO4)(2n). 2nH(2)O (4) (whose structure was previously reported) have been p erformed. Compounds 1 and 2 crystallize in the same trigonal space group, R (3) over barc; 3 is orthorhombic, space group Pbca. Complexes 1 and 2 are built of linear dpp-bridged chains which extend along threefold screw axes. The copper atom has in each case an elongated octahedral geometry with pyr azine nitrogen atoms in axial positions. The prominent feature of the cryst al packing is the supramolecular arrangement of six chains around a threefo ld inversion axis, creating channels housing the counter ions, and in the c ase of 1, also crystal water. In 3 the chain is zig-zag shaped and extends along a twofold screw axis. Counter ions and crystal water are situated in channels formed between four symmetry related chains. At room temperature ( r.t.) the X-ray results show a copper ion with a compressed octahedral coor dination geometry, pyrazine and pyridyl nitrogen atoms binding in equatoria l and axial positions, respectively. The low temperature X-ray studies of 3 show significant changes in the copper coordination geometry, strongly sug gesting that the apparent compressed geometry at r.t. is due to a dynamic J ahn-Teller distortion. The Cu . . . Cu separations across the bridging dpp at r.t, are, 7.133(1) (1), 7.228(1) (2), 7.005(1) (3) and 7.002(2) Angstrom (4). X-band ESR spectra of 1 and 2 exhibit the pattern of Cu(II) in elonga ted geometry (g(parallel to) > g(perpendicular to)> 2.0), whereas those of 3 and 3 exhibit inverse (g(perpendicular to) > g(parallel to) > 2.0) patter ns with a shoulder in the perpendicular signal. Complexes 1-4 exhibit a Cur ie law behaviour with very weak intrachain antiferromagnetic coupling, the relevant magnetic parameters being J = - 0.27 cm(-1), g = 2.11 for 1, J = - 0.17 cm(-1), g = 2.09 for 2, J = - 1.38 cm(-1), g = 2.15 for 3, and J = - 1.36 cm(-1), g = 2.14 for 4 (the Hamiltonian being (H) over cap= - JS(A)S(B )). (C) 2000 Elsevier Science B.V. All rights reserved.